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HD interfaces

Late last year, Intel announced that it plans to end support for the VGA and LVDS specifications on display interfaces, in favor of HDMI and DisplayPort. While this news primarily affects PC displays and consumer TVs, it will also have an impact on video and audio interfaces on professional displays, this month's topic.

Evolving need for video bandwidth

The primary interfaces for handling video on professional equipment are the analog RGB, YCbCr and CVBS (composite), and the digital SDI, HD-SDI and 3G-SDI; MPEG transport streams are carried by DVB-ASI as well. Because the display market is driven primarily by consumer display technology, equipment development tends to start in the consumer space, with “narrower” applications like broadcast and production following, albeit with professional specs and features.

Hence, consumer displays began with analog CVBS and VGA connections and evolved up to the analog/digital DVI, digital HDMI and now DisplayPort interfaces. At the same time, professional displays, needing to interface to professional equipment, started with analog and evolved up to SDI. But now, HDMI is beginning to appear on professional displays, especially for use with cameras. The growing use of DSLRs for production capture has similarly increased the need for HDMI interfaces.

Because the marketing of consumer equipment is sensitive to pricing (read: bill of materials), manufacturers during the product development process are faced with a constant struggle between adding more functionality and lowering cost. For this reason, a single “universal” interface would be preferable to the sea of connectors often seen on consumer equipment. Because much of the functionality of CE equipment is dependent on the state-of-the-art of silicon, manufacturers are thus motivated to work with integrated circuit manufacturers to push product evolution in the direction of interface simplicity/sophistication, and many of the same chips used in consumer equipment have found their way into pro monitors as well.


The mix of manufacturers wanting to move on from analog VGA interfaces includes AMD, Dell, Intel, Lenovo, Samsung Electronics LCD Business and LG Display. All of these have announced the desire to adopt scalable and lower-power digital interfaces into PCs. At the same time, low voltage differential signaling technology (LVDS) is being replaced with DisplayPort for digital interfaces as well. LVDS was developed as a low-power general-purpose digital interface standard that carries signals on a twisted pair within a device. Such an interface was thus used to transport digital video signals between, e.g., the motherboard and the flat panel for a laptop PC.

But DisplayPort is becoming a preferred choice over LVDS, according to Intel, because of power advantages, bi-directional communications capabilities and design efficiency benefits. In fact, Intel plans to end support of LVDS in 2013 and VGA in 2015 in its PC client processors and chipsets.

To carry video between equipment and flat-panel monitors, HDMI and DisplayPort are becoming preferred solutions. As of version 1.2, DisplayPort has a maximum data rate of 21.6Gb/s, can run on cables up to 15m in length and supports multiple color spaces including xvYCC, scRGB and Adobe RGB 1998. It is also capable of transporting multiple audio/video streams, supports bi-directional data transfer, has a global time-code (GTC) for audio synchronization and provides an AUX channel with a bandwidth of 720Mb/s, which can carry Ethernet, USB 2.0, DPMS and other types of data. When carrying video over an internal connection, a version of DisplayPort called Embedded DisplayPort (eDP) can be used.


HDMI encoding includes support for 4:4:4 RGB, as well as 4:4:4 and 4:2:2 YCbCr color spaces. The 4:4:4 format is 8 bits per component, and the 4:2:2 format can be up to 12 bits per component for greater color depth. Version 1.4 of the HDMI specification runs at up to 10.2Gb/s, supporting up to 1080p video, and adds support for three additional color gamuts.

In addition to xvYCC color space and Deep Color, this latest HDMI standard now offers native support for sYCC601, Adobe RGB color and Adobe YCC601 color. At the physical level, HDMI uses transition-minimized differential signaling (TMDS) instead of LVDS, which enables it to drive longer cables, but still at a low power consumption.

HDMI offers some functionality that does not exist with DisplayPort, such as support for Consumer Electronics Control (CEC) signals (used for remote control functions), electrical compatibility with DVI, and an Audio Return Channel (ARC) that simplifies cabling by allowing, for example, a display with a built-in tuner to send audio back to another device. Both HDMI and DisplayPort can carry audio as well, with up to eight channels of 192kHz, 24-bit uncompressed audio, as well as any of the common compressed audio formats, such as Dolby or DTS.

Both interfaces also support High-bandwidth Digital Copy Protection (HDCP), preventing the unauthorized copying of content on the interface. DisplayPort, however, can provide DPCP DisplayPort Content Protection (DPCP) as well. Although some manufacturers speak of “Wireless HDMI,” there is in fact no standard for such an interface, with proprietary products emerging that wirelessly interconnect HDMI sources and sinks. But, a multi-gigabit wireless DisplayPort specification is said to be in the works.


The USB interface has evolved over the years as well, to the point where version 3.0 can carry up to 4.5Gb/s serial data, which is enough for uncompressed 1080i, or 1080p60 at eight bits; 1080p60 at 12 bits is just outside its capability. Even the earlier version 2.0, at 480Mb/s, can carry compressed video, which has resulted in USB adapters for such a purpose. Wireless USB is also available, operating at a speed of up to 480Mb/s at distances up to 3m, and operating in 14 bands in the 3.1GHz to 10.6GHz frequency range.

But USB has always been a universal interface to be used with a wide variety of products, including those outside of the video and audio realm. For that reason, it is likely that HDMI and DisplayPort, with their many video-related capabilities, will remain the most practical (and hence widespread) technologies for use with video monitors.

Aldo Cugnini is a consultant in the digital television industry.

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